I know the guys over at http://www.PartsPak.com sell remanufactured and new units. Here is the link to the one for a 20″ 90 hp 4-stroke outboard.

http://www.partspak.com/productcart/pc/Yamaha-Remanufactured-Gearcase-Lower-Unit-75-80-90-100-HP-1999-2000-2001-2002-2003-2004-2005-4Stroke-20- for-Outboard-3840p7553.htm